Method of etching semiconductors



Dec. 6, 1966 D- T. KELLEY METHOD OF ETCHING SEMICONDUCTORS 2 Sheets-Sheet 1 Filed July 9, 1965 INVENTOR. Dale T. Kelley 1 M6.

Dec. 6, 1966 Filed July 9, 1965 D- T. KELLEY METHOD OF ETCHING SEMICONDUCTORS 2 Sheets-Sheet z INVENTOR.

DALE 'I'. KELLEY ATT'YS.

United States Patent 3,290,192 METHOD OF ETCHING SEMICONDUCTORS Dale T. Kelley, Phoenix, Ariz., assiguor to Motorola, Inc., Franklin Park, Ill., a corporation of Illinois Filed July 9, 1965, Ser. No. 470,693 5 Claims. (Cl. 15617) This is a continuation-in-part of the application filed June 5, 1962, Serial No. 200,250 now abandoned.

This invention relates generally to the art of etching, and more particularly to a method for chemically machining materials that can be etched or dissolved.

Many materials are difficult to machine by ordinary methods because of extreme hardness, abrasive qualities, unusual structure or similar reasons. While it has been known that certain chemicals or chemical solutions will react with or dissolve some materials more easily than they can be machined by conventional means, previous etching methods did not permit desirable control and selectivity.

Electrolytic etching methods, for example, are unsuited for etching non-conductors such as glass. Another limitation of electrolytic etching, besides the requirement that the work piece be capable of conducting significant current, is the determination of a suitable electrolyte. It is desirable that the electrolyte chosen be chemically stable during electrolysis and a good conductor of electricity. There is an additional problem in a wet system of elec trical isolation of the system as a whole and the individual elements. Another method of shaping materials, which do not machine well on standard machine shop equipment and tools, utilizes the cavitation phenomenon. This cavitation technique requires expensive equipment capable of producing high frequency vibrations. Since this process relies on the physical characteristics of the work piece and, at times, suspended matter in the surrounding fluid medium there is an inherent limitation as to the accuracy with which items may be produced.

Accordingly, it is an object of this invention to provide a method for machining certain materials that are difiicult to machine by ordinary methods.

It is another object of the invention to provide an improved method for machining materials regardless of electrical resistivity that can be etched or dissolved.

It is still another object of the invention to provide a means of shaping or forming a piece of material that can be etched or dissolved, which means is simple and low in cost.

A feature of the invention is the provision, in a method for machining materials that can be etched or dissolved, of the step of locallyheating the bath in which the material is immersed, at the particular area in which the etching or dissolving is desired, to raise the temperature of the bath above the practical active point at that area.

Another feature of the invention is the provision, in a method for machining materials that can be etched or dissolved, of the use of heaters or tools of particular forms .and shapes to achieve a desired type of etching or dissolving.

A further feature of this invention is the provision, in a method for machining materials that can be etched or dissolved, 'for elevating the temperature of particular locations in a bath by controlling the temperature of the work piece so that selected shapes may be obtained.

In the drawing:

FIG. 1 is a partial view of apparatus to perform the method of the invention;

FIG. 2 is a half sectional view of the tool used in the method of FIG. 1;

FIG. 3 is a view of a second application of the method;

FIG. 4 is a schematic view of a third application of the method;

FIG. 5 is a schematic view of a fourth application of the method;

FIG. 6 is a perspective view of a chuck assemble used in another embodiment of this invention;

FIG. 7 is a cross sectional view of the apparatus used in conjunction with the chuck of FIG. 6;

FIG. 8 is a cross sectional view of an etched Work piece;

FIG. 9 is a cross sectional view of an etched work piece; and

FIG. 10 is a cross sectional view of an etched work piece.

An et-chable work piece may be machined in accordance with this invention by placing the work piece in a bath which is maintained below the practical active temperature of the solution. The bath should be composed of an etching fluid which will etch the work piece at an increased rate with increasing temperature. The temperature of the bath is then raised locally at selected locations near the work piece.

The invention comprises the maintaining of a bath of etching fluid or solvent at a sufliciently low temperature to effectively arrest the action of the solution on the particular material to be machined. The material is then submerged in the bathand a small tool-like heating device is brought near the particular point on the material at which machining is to take place. The heating device will raise the temperature of the fluid at that point sufliciently to cause an etching or dissolving action. Thus, the heater acts as a cutting tool although it may or may not make actual contact with the material, enabling the material to be etched to a desired form and shape.

It is well known that the action of certain liquids or gasses on some solid materials will cause an etching or corrosive action. This is due to a chemical reaction taking place between the fluid and the solid material. Most such chemical reactions exhibit tremendous increases in reaction rate with moderate increases in temperature. The variation in rate with temperature is exponential somewhat in the manner illustrated by the simplified expression where R is the time rate of reaction, B and C are substantially constant values, e is the base of natural logarithms, and T is the Kelvin temperature. Since the rate of etching increases as the etching fluid temperatrues increases, etching rates may be substantially controlled by controlling the temperature of the etching fluid. When the fluid is cooled to a sufficiently low temperature, the chemical reaction is inhibited or prevented entirely. In most cases, control of etching and reaction rates by controlling temperature is possible.

Etching involves a broad group of similar chemical reactions involving, generally, surface oxidation and reduction. These reactions, which are sometimes complex, are generally well known in the field of chemistry for most etchable materials. The etching solutions generally consist of buflered acids or bases. A few of the commonly known etching processes are; the etching of glass (SiO )with hydrofluoric acid, the etching of steel with hydrochloric and sulphuric acid to remove undesirable oxides, and the etching of silicon with a solution of nitric, hydrofluoric, and acetic acids. In these and similar chemical etching processes, the rate of reaction is controlled .by a suitable combination of the amount of buttering and the solution temperature. The determination of a suitable etchant for a desired work piece is normally made by consulting one of the many available listings of the physical properties of substances such as that found in Chemical Engineers Handbook, John H. Perry, editor, Third Edition, 1950, pages 110-148.

A common use of chemical solvents is for the removal of some undesirable material. Another use is in the fabrication of shaped items in which a suitable solvent is used to permit the joining of two pieces. Again, there has been no attempt to differentiate the rate of reaction of the chemicals on various portions of the work piece. A similar result is possible in the case of a solvent acting upon a solid material which is soluble in that liquid. It is well known that when a solvent solution becomes saturated, materials ordinarily soluble in that solvent will no longer dissolve. The saturation point of a particular solvent is a function of temperature as well as of concentration of dissolved material therein, and a solvent may be of such a concentration as to have a saturation point at a given temperature, below which an ordinarly soluble material will be insoluble. The listings of physical properties of substances cited above will disclose the many solvents which are available for shaping work pieces.

Although etching involves a chemical reaction and dissolving has more physical characteristics, for the purposes of shaping work pieces, the properties are so similar that they will be considered together in this invention. For the purposes of the remainder of the specification and the claims, the terms etch when used as a verb, and etching are defined as referring to the rernoval of material from a body or object by non-electrolytic chemical action including the physical chemical action of dissolving. The terms bath, etch bath, etching solution, ecthing fluid, and etch (where used as a noun) are defined as referring to solvents as well as chemically active fluids.

The terms machine and machining are to be taken as referring to the forming of a work object to a particular size or shape by removing material therefrom aided .by a tool or similar instrument.

Referring now to FIG. 1, a work object 11 which may be a glass rod, is shown immersed in an etching solution 13 which may be dilute hydrofluoric acid for example. In this particular application of the method, the machining operation is similar to that of the ordinary lathe in that the work object 11 is rotated in the direction of the arrow by suitable drive means (not shown), and a tool 15 having a heater portion 17 thereon is used to shape the work object 11. Removal of material from work object 11 takes place when fluid 13, which has been cooled below the practical active temperature thereof (that is, below the practical reactive temperature in the case of a chemical reaction type etch, and below the saturation temperature in the case of a physical action type etch), is heated by tool 15. Some reaction is permissible and will probably occur at most operating temperatures and concentrations. The amount of material removed, however, would be negligible until temperature is locally raised by tool 15 to the point where material is removed in practical quantities. The term practical active temperature refers to this latter point. Cooling of the fluid might be accomplished, for example, by pumping water from an ice and water tank, not shown, through exchange coils 18 immersed in fluid 13. Coils 18, of course, must be of a material which is unaffected by fluid 13. The temperature of work object 11 may be maintained at a constant value by using the fluid 13 as a coolant, or by some other means as by heat sinking the work object or circulating water through it. Concentration levels can be maintained by a number of means such as using a large volume of etching fluid, buffering, or continuously adjusting the fluid to maintain a monitored concentration. In this and in many other cases, circulation or agitation of the fluid may be useful in maintaining a more uniform fluid temperature and concentration.

Tool 15 is positioned so that heater portion 17 is sufficiently close to work object 11 to locally raise the tem-' perature of the fluid and cause etching on the work object in the area to which the tool is proximate. Concentration and temperature levels between tool 15 and work object 11 are maintained due to the rotation of the Work object which continuously brings fresh etch between itself and the tool. Thus, tool 15 may be moved in any desired direction to form a particular shape in work object 11, although it need not make contact with the material. For example, the tool shown can be used to form the radius R, and is also suitable for removing ma terial along the length L. Only that portion of work object 11 desired to be machined must necessarily be immersed in the etching fluid providing, of course, a constant temperature of the work object is maintained, and it should be noted that the drawings are merely intended as schematic representations.

Tool 15 may be of any desired shape or size suited to a particular machining purpose and may be heated by any practical method. One type of tool which may be used in an operation such as that depicted in FIG. 1 is shown in FIG. 2. Tool 15 has a heater portion 17 which may be composed of a suitable heat conductive material upon which the etching fluid will have little or no effect. For instance, with a bath of hydrofluoric acid for machining a work object of glass, a tool of copper could be plated with gold several times, bu-rnishing between plates to close any openings in the plating. The tool could also be coated with platinum. In either case, it will resist hydrofluoric acid. A center tube 19 is inserted into a chamber 21 in the heater portion 17 of the tool. Spaced from tube 19 and fully surrounding it is outer tube 23. A hot liquid, such as hot water, is pumped through tube 19 into chamber 21 in which a heat exchange takes place, raising the temperature of heater portion 17. The water is then returned from chamber 21 through tube 23 to be reheated or discarded. A protective cover 25 surrounds tube 23 thereby protecting it from contact with the etching fluid. Protective cover 25, of course, is of a material which is unaffected by the etching fluid, and further may be used as thermal insulation to prevent temperature rise in the etching fluid adjacent the tubes 19 and 23.

, The method of applicants invention has many and varied applications, one of which is shown in FIG. 3. This operation is most closely analogous to that of a milling machine. A work object 11 is immersed in an etching fluid '13. A tool 15 having a heater portion 17' thereon is brought into proximate relationship with the particular area on work object 11' which is desired to be machined. As the etching fluid 13 is heated locally in that area, etching occurs, removing material from work object 11 in the desired manner according to the motion of tool 15'. If necessary for some cases, the tool 15 and heater 17 may be rotated as shown by the arrow to bring more fresh etching fluid between the heater 17' and the work object 11', thereby producing a flushing action which removes bubbles and otherwise maintains optimum etching conditions as will be further dmcribed.

It may, under some circumstances, be desirable to have the tool make contact with the work object to heat the work object. Such an application of the invention is depicted in 'FIG. 4. A tool 31 having a heater portion 33' thereon is brought in contact with a work object 35 immersed in an etching fluid 13. Heater portion 33 is formed in a desired raised pat-tern causing removal of material from a glass work object 35 only in certain are-as according to the form or pattern of heater portion 33. If required, the tool 31 and heater portion 33 can be rolled back and forth relative to the work object until the pattern design is fully formed on the work object. In certain applications of this type where there is .apt to be limited or slow relative motion between the work object and the heater, it may be worthwhile to provide a suitable means to efficiently control or remove bubbles formed during etching since they inhibit the speed and quality of the etching. Ultrasonic agitation of the etch, work object or heater is an example of a way in which bubbles could be removed.

From the above discussion it is readily apparent that other forms of tools and applications thereof may be designed to utilize this method and to fit the particular needs of the operation. For instance, equipment may be designed to perform the functions of drilling, sawin broaching, grinding, lapping, polishing, etc. The work object or fluid may be heated in some other manner than by the use of the described tools. For example, heating may be accomplished by the use of radio frequency signals, etc. With proper etching solutions, very low solution temperatures, and tool temperatures approaching the boiling point of the solution, extremely fast roughing speeds may be attained. To do very accurate and controlled work, and work where the etched material closely matches the contours of the tool, it is only necessary to lower the temperature of the tool so that the temperature difference between tool and etching fluid is not too great, and then operated at suitable feeds and speeds.

In some forms of the equipment, it may become necessary to agitate (vibrate, rotate, etc.) the etching tool as well as the solution in order to attain a more controlled etching action. Various procedures may be developed which are suited to the particular tool .and etching op eration being used to bring fresh solution between the tool face and the material being etched. One way in which this may be accomplished is illustrated by the tool 37 in FIG. 5 which is designed to cut out a disc shaped part 38 of work object 39. The working (heated) race of the tool, similar to a housewifes cookie cutter, is agitated or vibrated up and down to flush fresh etch between the tool face and the surface being etched. Often gas bubbles will form on the work object and since these bubbles tend to cause unsatisfactory etching, it is desirable to remove them. Agitation of the tool such as is shown in FIG. 5, will also help to keep the work object bubble free. Other possible ways of agitating or vibrating the tool will be readily apparent to those skilled in the art. The solution may be agitated in a number of diverse ways, for example, ultrasonically by some sort of transducer arrangement.

In another embodiment of this invention, FIG. 6, a work piece of silicon 51, 6 mils thick, has been mounted on a two piece chuck 53 so that only one surface of the silicon 51 will be exposed. This chuck 53 is constructed so that the temperature of the silicon '51 may be controlled on the surface which is not exposed. The silicon 51 has had a mask 55 applied of a well known etch resistant material to produce a desired surface configuration 54. In this embodiment KMER, a commercial etch resistant material manufactured by the Kodak Company was used. Another etch resistant material, polyvinylohloride, was used to construct the back portion of the chuck 53. The front piece '60 of the chuck 53 is metal which has been platinum or gold plated to make it etch resistant. These materials were selected after careful consideration was given to the etching solution and temperatures involved.

The temperature inside the chuck 53 was maintainedat 30 C. by a constant flow of water through the circulating tubes 62. The platinum or gold plated face 60 acts as a heat exchanger with the silicon 51 and maintains the unexposed surface of the silicon 51 at a relatively constant temperature. When this chuck 53 with the silicon 51 mounted thereon was immersed in the etching bath, 70, FIG. 7, which was maintained at about l5 C., a temperature gradient was established through the thickness of the silicon 51 with the exposed surface having a temperature lower than the unexposed surface.

The etching bath consisted of one part nitric acid (67 to 71%) three parts hydrofluoric acid (48%) and one part acetic acid (99.5%). A container 72 holding this etching bath 70 was placed in a constant temperature bath 75. In this embodiment the solution 75 consisted of ice, water, and rock salt in a container 76 mounted on an agitator table 77. The agitator table 77 aids in establishing and maintaining a constant temperature. The agitator table 77 also prevents the development of undesirable concentrations at the surface of the silicon 51. The silicon 51 mounted on the chuck 53 was then immersed in the etching bath 70 for four minutes, removed from the etching solution and thoroughly rinsed and dried. For a piece of silicon six mils thick, a tapered configuration :FIG. 8 was obtained. If this silicon 51 is left in the etching bath 70 for an extended period of time an undercut configuration FIG. 9 will result because of the higher temperature of the solution at the heat exchanger face 60.

In still another embodiment of this invention utilizing the same equipment as described in the preceding embodiment, with the following changed conditions, an etching bath 70 of four parts nitric acid (6771%) and one part hydrofluoric acid (48%) was maintained at a temperature of 45 C. by a heating source 6-2. The tempera-ture of the chuck 53 was maintained at 25 C. Thereby, a temperature gradient the reverse of the preceding embodiment was established.

A piece of silicon 51, masked with wax 55, 8 mils thick was placed in the etching bath 70 for 9 minutes and 30 seconds, removed from the bath 70 and thoroughly rinsed, and dried. A reverse taper FIG. 10 was obtained on the etching profile with substantial undercutting in the area of the mask 55. Thus, it is possible to obtain various degrees of taper by adjusting the temperature of the solution and the work piece and the etching time.

I claim:

1. A method of etching, including the steps of mounting a silicon work piece having a surface masked with an etch resistant material on holding means, immersing said work piece and holding means in an etchant to expose said masked surface thereto, maintaining said work piece at .a temperature different from that of said etchant during at least a part of said immersion, and agitating said etchant during immersion.

2. A method of etching a silicon work piece according to claim 1 in which said work piece is rinsed after removal afirom said etohant.

3. A method of etching a silicon work piece according to claim 1 in which said etc-hant is maintained at a higher temperature than said work piece.

4. A method of etching a silicon work piece according to claim 1 in which said etchant is maintained at a lower temperature than said work piece.

5. A method of etching a silicon work piece according to claim 1 in which said etchant comprises between about 1 and 5 parts nitric acid; 1 and 5 parts hydrofluoric acid; and 0 and 5 parts acetic acid.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 1/ 1958 Great Britain.

ALEXANDER WYMAN, Primary Examiner.

JACOB STEINBERG, Examiner. 

1. A METHOD OF ETCHING, INCLUDING THE STEPS OF MOUNTING A SILICON WORK PIECE HAVING A SURFACE MASKED WITH AN ETCH RESISTANT MATERIAL ON HOLDING MEANS, IMMERSING SAID WORK PIECE AND HOLDING MEANS IN AN ETCHANT TO EXPOSE SAID MASKED SURFACE THERETO, MAINTAINING SAID WORK PIECE AT A TEMPERATURE DIFFERENT FROM THAT OF SAID ETCHANT DURING AT LEAST A PART OF SAID IMMERSION, AND AGITATING SAID ETCHANT DURING IMMERSION. 